What's the use of the jar on the roof of Deyang residential building in Sichuan?

There is a small water tower in my home. Generally, there used to be pressure wells (groundwater) at home. Pump it into the water tower for storage, and then pump it after use.

I really can't save much water, because I pump groundwater and use a small pump. So don't use too big a water tower, that bucket of water is enough for a family to use for a long time. They are sometimes used for bathing and washing dishes. . . Usually it is not widely used.